The present invention relates to an optical device which can be configured as a source which emits single or multiple photons at a predetermined time or which can be configured as an optical switch.
As an optical source, the device is capable of emitting pulses of n-photons where n is controllable and is an integer of at least 1. More specifically, the present invention is concerned with such a device which has means for directing photons such that they can be efficiently collected by an external medium such as a fibre optic cable and a method of making such a device. The present invention can be used to increase the efficiency of any light emitter.
There is a growing need for single photon sources for use in optical quantum cryptography where, for example, a security key for an encryption algorithm is delivered by a stream of single photons which are regularly spaced in time. It is essential for the security of this technique that each bit is encoded on just a single photon. This is because an eavesdropper trying to intercept the key will be forced to measure and thereby alter some photons while reading the key. Therefore, the intended recipient of the key can tell if the key has been intercepted.
Such a source is also useful as a low-noise source for optical imaging, spectroscopy, laser ranging and metrology. Normal light sources suffer from random fluctuations in the photon emission rate at low intensities due to shot noise. This noise limits the sensitivity of many optical techniques where single photons are detected. A single photon source which produces photons at regular time intervals has a reduced shot noise.
Previously, single photon sources have been envisaged by strongly attenuating a beam emitted from a pulsed laser source. Such a source is shown in FIG. 16. Emitted beam 101 is fed through an attenuator 103. The attenuator is configured so that the average number of photons transmitted (0 is about 0.1. Since photons are indivisible, this means that about 10% of the periods will contain a single photon, while 90% of them are empty.
This method has two serious drawbacks. First, there is still a small probability (μ2/2) of finding more than one photon within one period. A significant number of these multi-photon emissions would seriously impinge on the security of the device for quantum cryptography. The second problem is that most of the periods are empty and hence carry no information. Thus, the time which it takes to send the key is increased, and also, the maximum distance over which it can be transmitted is limited. The distance limitation problem arises from the fact that optical quantum cryptography is only effective when the rate of detected photons is much higher than the ‘dark count’ rate of the detector. Therefore, a system with a high emission rate source can tolerate more transmission loss, and therefore a longer transmission distance medium before being overwhelmed by the detector dark count.
More promising attempts at making such a single photon detector have concentrated on controlling the generation of photons. For example, the inventor's unpublished UK patent application number 9927690.9 and J Kim et al, Nature, 397, p 500 (1999) have both suggested single photon emitters using a quantum box to control the recombination of electrons and holes. In the inventor's unpublished UK patent application, the emitter comprises a quantum dot having a first confined energy level capable of being populated by an electron and a second confined energy level capable of being populated by a hole, the emitter also comprises supply means for supplying carriers to the said energy levels, wherein the supply means are configured to supply a predetermined number of carriers to at least one of the energy levels to allow recombination of a predetermined number of carriers in said quantum dot to emit at least one photon. Foden et al, Phys. Rev. A. 62 011803 (R) (2000) have suggested a device where recombination of electrons and holes is controlled using Surface Acoustic Wave (SAW) to transport carriers through a quantising means prior to recombination.
However, although some advances have been made in controlling the generation of the photons, there is still a problem in that it is very difficult to efficiently collect photons generated by such a source.
In most single photon emitters, the photons are emitted essentially isotropically by the device. Thus, only a small fraction of the emitted photons can be collected by an optical fibre or the like and subsequently used. This problem, compounded by the fact that some of the known photon sources have poor emission efficiency, means that the total efficiency of the device can be as little as 0.01%.
The present invention attempts to address the above problems by using a three dimensional optical cavity, which confines the emitted photon field in three dimensions and allows the emission direction of photons leaving the cavity to be controlled.
Gerard et al, Applied Physics Letters, 69, p 449 (1996) describe an experiment which uses a quantum dot to probe the photonic microstructure of a three dimensional cavity which is formed in an etched pillar. This so-called pillar cavity is formed from a stack of alternating semiconductor layers which form Bragg reflectors provided above and below a light emitter. The stack of layers is then etched to form a pillar. The reflective etched outer surface of the pillar provides some lateral confinement However, this method of forming such a three dimensional cavity is flawed since the device exhibits poor confinement in the lateral direction due to the low reflectivity of the etched walls. Smith et al, Electronics Letters 35 228 (1999) have studied optical confinement using a photonic crystal.